Loomio seems to have added a new setting, NOTIFICATIONS_EMAIL_ADDRESS, to
define what address transactional emails should come from. 

If you don't set it, it will default to notifications@$MAIL_DOMAIN, which is
unlikely to work in many cases.

If you find that transactional emails aren't working, try setting
NOTIFICATIONS_EMAIL_ADDRESS to the same value as SUPPORT_EMAIL,
HELPER_BOT_EMAIL, or SMTP_USERNAME.